Steel cords for reinforcing breaker or belt plies in a rubber tire are well known in the art.
U.S. Pat. No. 4,408,444 discloses a M+N construction, and more particularly a 2+2 construction. This cord has two groups of filaments, a first group with M, preferably two filaments and a second group with N, preferably two filaments. This cord, at least in its 2+2 embodiment, has the advantage of full rubber penetration whether brought under tension or not. However, this cord construction suffers from the drawback of having a relatively poor fatigue limit and too great a cord diameter.
In an attempt to mitigate these drawbacks, EP-B1-0 466 720 proposes a similar but different M+N construction. The difference is that the filaments of one group have a filament diameter which differs from the filaments of the other group. The result is an increase in fatigue limit and, sometimes, a decrease in cord diameter for the same reinforcing effect.
M+N constructions with difference in filament diameters are, however, difficult to process during tire manufacturing, particularly in an automated system. Filaments with a difference in diameter have different saturation levels of residual torsions. The resulting cords are subject to flare. The cords are less stable and the integration of such cords in rubber plies leads to tip rise of the rubber plies, i.e. one or more edges are lifting up.
When applying torsions to a steel cord or a steel filament, the first observed phenomenon is linear, i.e. the number of residual torsions is equal to the number of applied torsions. Further increasing the number of applied torsions leads to an increase of residual torsions but not to the same degree: in decreasing amounts. In other words, a saturation phenomenon is observed. As soon as there is no increase anymore of residual torsions, the saturation level of residual torsions has been reached.
The saturation level of residual torsions of a steel filament is dependent upon the material of the steel filament, the tensile strength of the steel filament and, especially, upon the diameter of the steel filament.
In order to cope with the problem of tip rise, WO-A1-2012/128372 proposes a 2xdc+Nxds construction where the filament diameter of the core group dc is greater than the filament diameter of the sheath group ds and where the two core filaments are plastically deformed to such a degree that they form a wave with such an amplitude that the core steel filaments get well anchored by the rubber in the ultimate rubber ply. This anchorage hinders any negative effect of residual torsions and lowers the tip rise of any reinforced rubber ply.
However, the 2xdc+Nxds of WO-A1-2012/128372 suffers from flare and risks to become a less robust or less stable construction.
The term ‘flare’ refers to the phenomenon of spreading of the filaments ends or the strand ends after cutting of the steel cord or steel strand. A steel cord without flare does not exhibit this spreading, the filaments or strands remain more or less in their position after cutting.
Patent applications JP-A-2013199194, JP-A-2013199193, JP-A-2013199191, JP-A-2013199717, JP-A-2013199195, JP-A-2013199190, JP-A-2013199189 all disclose 2xdc+Nxds steel cord constructions but they do not offer a solution of the problem of flare and neither a solution for the too great a wave of the core steel filaments.
JP-A-06-306784 discloses a way of manufacturing a 2 (core)+2 (sheath) steel cord construction by means of a double-twister where used is made of a turbine or false twister. The core steel filaments and the sheath steel filaments have the same diameter.
U.S. Pat. No. 5,487,262 discloses a method and device for making a steel cord where use is made of two false twisters in sequence.